An imaging apparatus using optical coherence tomography (OCT) in which interference due to low coherent light is utilized (hereinafter also referred to as OCT apparatus) is currently put into practical use. The OCT apparatus can take a tomographic image of an object to be inspected with high resolution.
In an OCT apparatus, light from a light source is split into measuring light and reference light by a beam splitter or the like. The measuring light irradiates an object to be inspected, e.g., an eye, via a measuring optical path. Then, return light from the object to be inspected is combined with reference light and is guided to a detector as interfering light via a detection optical path. The return light described here refers to reflected light and scattered light including information about an interface in the light irradiation direction with respect to the object to be inspected. The interfering light of the return light and the reference light is detected by the detector and is analyzed so that a tomographic image of the object to be inspected can be obtained.
Here, in order to acquire a tomographic image with high resolution in the OCT apparatus, it is necessary to match the dispersion amount of an optical system in the measuring optical path to that in a reference optical path. When the dispersion amounts are different, blurring occurs in a tomographic image, and the resolving power in a depth direction is degraded. Therefore, in the OCT apparatus, there has been known a configuration in which a dispersion compensating glass is arranged on the reference optical path to compensate for the dispersion with respect to the reference light. Further, in order to address the difference in dispersion caused by a machine difference among various optical elements arranged in the apparatus, there has been known a device configured to compensate for the difference by signal processing (see Patent Literature 1).
In the configuration disclosed in Patent Literature 1, through arrangement of the dispersion compensating glass in the reference optical path, the dispersion that occurs when the measuring light reciprocates between an eye to be inspected and a scanning optical system is compensated for with respect to the reference light. Further, through determination of a phase shift amount from the tomographic image, the difference in dispersion between the measuring light and the reference light is compensated for by signal processing.
Meanwhile, in fundus inspection, it is required to image a wide range so as to include a peripheral portion of a fundus, or image a specific part of the fundus with high resolution depending on the condition. Here, by changing an objective lens to an objective lens having a different focal length, the imaging range and the resolving power in a lateral direction can be switched. That is, through use of an objective lens having a shorter focal length, a wider range can be imaged with a low magnification, and through use of an objective lens having a longer focal length, a narrower range can be imaged with a high magnification and high resolution.
However, when an objective lens arranged on the measuring optical path is changed, the dispersion amount of the measuring optical path is varied due to the difference in dispersion between the objective lenses before and after the change, with the result that a difference in dispersion is caused between the measuring optical path and the reference optical path. When the difference in dispersion is caused, blurring occurs in a tomographic image as described above, and the resolving power in a depth direction is degraded. When the thickness of the dispersion compensating glass is changed for each objective lens in order to correct the difference in dispersion, a mechanism for this purpose is separately required, resulting in a complicated apparatus. Further, when the difference in dispersion is corrected by calculation processing, the calculation processing is performed for each objective lens, and hence a calculation load increases.